Electronic stencil cutter with cutting signal pulse width inversely proportional to the original intensity

ABSTRACT

An electronic stencil cutter for reproducing an original on a stencil. A photomultiplier scans the original and produces an output which is directly proportional to the intensity of the pattern on the original. The output of the photomultiplier is applied to a stylus control signal generator which produces a pulse train output having pulse widths inversely proportional to the intensity of the pattern on the original. The output of the stylus control signal generator is applied to a power amplifier which produces a stylus current inversely proportional to the intensity of the pattern on the original. The stylus current is used to generate a spark which produces the image on the stencil. The stylus control signal generator includes a capacitance, an oscillator and a switching device coupled in such a manner that the OFF period of the switching device is inversely proportional to the current through the photomultiplier.

United States Patent 1 Egerton, Jr. R

[ ELECTRONIC STENCIL CUTTER WITH CUTTING SIGNAL PULSE WIDTH INVERSELY PROPORTIONAL TO THE ORIGINAL INTENSITY [75] Inventor: McKenny W. Egerton, Jr., Owings Mill, Md.

I73] Assignce: Instrument Corporation of America,

Timonium, Md.

[22] Filed: July 5, 1972 [211 App]. N0.: 269,203

{ WHITE mm Apr. 2, 1974 Primary ExaminerRaymond F Cardillo, J r. Attorney, Agent, or Firm-Le Blanc & Shur [57] ABSTRACT An electronic stencil cutter for reproducing an original on a stencil. A photomultiplier scans the original and produces an output which is directly proportional to the intensity of the pattern on the original. The out put of the photomultiplier is applied to a stylus control signal generator which produces a pulse train output having pulse widths inversely proportional to the intensity of the pattern on the original. The output of the stylus control signal generator is applied to a power amplifier which produces a stylus current inversely proportional to the intensity of the pattern on the original. The stylus current is used to generate a spark which produces the image on the stencil. The stylus control signal generator includes a capacitance, an oscillator and a switching device coupled in such a manner that the OFF period of the switching device is inversely proportional to the current through the photomultiplier.

7 Claims, 5 Drawing Figures PROCESSOR PHOTO- MULTIPLIER I4 CARRIAGE MOTOR 5 V smus 0 R e N A L L s T EN c L ROTATING DRUM 6 STATE 2 FIG. 3/!

STATE 1 Iw FIG. 3B

H036 OFFL U LLJ ELECTRONIC STENCIL CUTTER WITH CUTTING SIGNAL PULSE WIDTH INVERSELY PROPORTIONALTO THE ORIGINAL INTENSITY BACKGROUND OF THE INVENTION machine comprises a rotating drum which carries the original on one half and a blank stencil on the other half. The original is scanned by a photomultiplier and the output of the photomultiplier is applied to a processor. The processor output is used to control a spark generated by a stylus. The spark creates a pattern of holes through the stencil corresponding to the pattern of dark areas on the original. The output of the photo- This invention is related to an electronic stencil cutter for reproducing an original on a stencil. A photomultiplier scans the pattern on the original and produces an output which is directly proportional to the intensity of the pattern. The output of the photomultiplier is applied to a processor which includes a stylus control signal generator. This generator comprises a capacitance which could, for example, be the stray camultiplier is proportional to the intensity of the origi- I nal, while the greater the spark produced by the stylus the greater the density or darkness of the image on the stencil. Therefore, the output of the processor must be inversely proportional to its input. 7

In one type of prior art system,the processor produces a full. amplitude spark if the pattern on the original is darker than a certain shade of grey, and no spark at all if it is lighter; In other more versatile machines, a grey scale can be reproduced by mo'dulatingthe spark intensity. This is done by'm odifying the photomultiplier signal with a non-linear network comprising diodes and resistors. The modification is required due' to the in verse proportionality between the input and output of the processor. The output of the network is used to control the amplitude of an oscillatory signal by means of a balanced modulator. This circuit is very complicated, and expensive. Further, the machines of this type require a linear power amplifier to drive the stylus. An amplifier of this type dissipates-a great amount of power. I

SUMMARY OF THE INVENTION It is an object of this invention to provide an electronic stencil cutting machine which eliminates the complexity in theprocessor used in prior art machines.

It is another object ofthis invention to provide an electronic stencil cutting machine which has a processor which can drive a power amplifier comprising a simple switching device dissipatinglittle power, rather than a linear amplifier as found in the prior art.

It is still another object of this invention to provide an electronic stencil cutting machine in which the processor output is a'pulse train, wherein the width'of the pulses is varied in accordance with the output of the photomultiplier. I

It is still another object of this invention' to provide means in the processor for varying the black level of the pattern on a stencil.

pacitance of the cable which is coupled to the photomultiplier. The capacitance is charged by current through the photomultiplier. An oscillator which switches between two states is coupled to the capacitance along with a switching means. The circuit is arranged s'uch that the switching'device is ON when the oscillator is in one of the states and also when the oscillator is in the other of its states and the capacitance is fully charged. The switching device is OFF when the oscillator is in the other of the two states and the capacitance is not fully charged. In this manner, the OFF period of the switching means is inversely proportional to the current through the photomultiplier means, since the charging of the capacitor is controlled by the current through the photomultiplier. For example, when the pattern on the original is very light or white, the intensity detected by the photomultiplier is great. Therefore, the photomultiplier current is high and the capacitor is charged rapidly. Since the capacitor charges rapidly the OFF period of the switching means is short. The output of the switching device is connected to the stylus circuit power amplifier and thereby controls the stylus current. It can be seen that the output of the photomultiplier, which'is proportional to the intensity of the light, is inversely proportional to the OFF period of the switching device. In this manner, the spark produced by the stylus, which controls the density of the stencil, is inversely proportional to the intensity of the original. A resistance may be connectedin parallel with the capacitance in the 'styluscontrol signal generator for varying the charging rate on the capacitor means. This variation affects the level of black on the stylus. If the resistance is used, the inverse proportionality of the OFF period of the switching device to the current through the photomultiplier is modified to become logarithmic. I

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a stencil cutter according to the present invention.

FIG. 2 is a schematic diagram of the processor used in the electronic stencil cutter in accordance with the .preferred embodiment of the present invention. a

FIG. 3a is a waveform of the output of the oscillator circuit used in the preferred embodiment of the present invention.

FIG. 3b is the waveform of the voltage across the capacitance used in the preferred embodiment of the present invention.

FIG. 30 is a waveform of the output of the switching device used in the preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, original 2 and stencil 4 are mounted on a rotating drum 6. Photomultiplier 8, lens 10 and stylusg'l2'are mounted on carriage l4, which is moved by motor 16 from left to right with respect to ro- 3. tating drum 6. The intensity of the pattern or image on original 2 is sensed by photomultiplier 8 and the output of the photomultiplier is applied to processor 18. The processor produces an output which is inversely proportional to its input and applies the output to stylus 12. The stylus generates a spark which perforates stencil 4. In this manner, a high intensity image on the original produces a light image on the stencil and a low intensity image on the original produces a dark or dense image on the stencil.

The schematic diagram of. processor 18 is shown in FIG. 2. The output of photomultiplier 8 is connected to the input of stylus control signal generator 20. This generator includes an oscillator circuit 22 connected to terminal 24 through a capacitor 26. Also connected to terminal 24 is a capacitance 28 which may be, for example, the stray capacitance of the cable from the photomultiplier. The base of switching device O1 is also connected to terminal 24.

Referring to FIG. 3, when the output of oscillator Cir-- cuit 22 switches from its first state to its second state, the voltage at terminal 24 is increased as shown in FIG. 3b. As photomultiplier 8 draws current as a function of the intensity of the pattern on original 2, capacitance 28 is charged in the negative direction as shown in FIG. 3b. When the voltage across capacitance 28 reaches Vc at point Al the capacitance is fully charged. Switching device O1 is biased so that it turns ON when the base potential reaches -Vc. When the voltage on the base of O1 is above Vc, the switching device remains OFF. This is illustrated in FIG. 30.

If the photomultiplier senses a moderate intensity or grey on the original, the capacitance 28 becomes fully charged at point A1 of FIG. 3b. If, however, the intensity of the original is greater, the photomultiplier 8 will draw more current and therefore the capacitance 28 will charge faster and become fully charged in a shorter period of time. If the intensity of the original is very small, that is, the original is dark, then the photomultiplier 8 draws less current and the time required to charge capacitance 28 is greater. These conditions are illustrated respectively as pulses A2 and A3 in FIG. 3b. It can readily be seen in viewing FIGS. 3a, 3b and 30 that the OFF period of the switching device O1 is inversely proportional to the amount of current through the photomultiplier. Since the amount of current through the photomultiplier is directly proportional to the intensity of the pattern on the original the OFF period of the switching device is inversely proportional to the pattern on the original.

Variable resistance network 30 is provided to balance out some small fraction of the photomultiplier current. This control is most useful when the darkest areas of the original are not very dark but are to be reproduced as black, and is hence termed the black level control.

If a resistance such as resistor 25 is connected from base to emitter of Q1, the charging curve of capacitor 28 is modified from approximately straight line as shown in FIG. 3b to exponential, and thus the processor law from inverse proportionality to logarithr'i iic. The use of this resistance is advantageous because the characteristics of the stencil and mimeograph process cause an error in the grey scale actually obtained. The error increases the contrast and thereby produces a more pleasing copy. If extreme accuracy is required,

then the resistance can be adjusted to produce an accurate rather than pleasing copy.

The output of switching device Q] is amplified by Q3 and applied to gate 32. The output of gate 32 is applied through gate 34 to transistor Q4 which along with O5 provides a necessary gain to producethe' stylus current.

Logic gate 32 is included to prevent t, the OFF period of switching device Q1, from exceeding percent of T, the period of oscillator 22. This condition occurs if the original is totally black and the photomultiplier current falls to 0.

Positive-negative switch 36 is used for reversing the image of the original produced on the stencil. When a positive image is required, the output of switching device Ql is applied to the base of transistor Q2. However, when a negative image is required, the output of switching device O1 is applied directly to the base of transistor Q3. A negative image might be used when, a blueprint having a dark background and white lines is to be copied as a pattern having a light background and black lines.

Transistor O5 is the output stage of the power amplifier for producing the stylus current. Transistor O5 is driven between cut-off and some point less than saturation. Although Q5 does dissipate some power, the amount of power dissipated is relatively small compared to the power dissipated in linear amplifiers used in the prior art. Resistor 38 may be used to adjust the stylus current to suit different types of stencil stock. As an alternative,'a simple switching device may be used in lieu of the power amplifier shown, and thereby further reduce the amount of power dissipated. v

Start-stop circuit 40 includes transistors Q6 and O8 and light responsivetransistor Q7. This circuit also includes a logic circuit comprising gates 42, 44, 46, 48, 50 and 34. The start-stop circuitfunctions to start and stop the stencil cutter and to interrupt the stylus current, when the stylus crosses a paper hold-down clamp.

Amplifier S2 is apower amplifier which supplies high voltage for the photomultiplier and low voltage for the lamps that illuminate the original. The supply voltage V4 to power amplifier 52 is variable, and may be used to adjust the gain and thereby control the light level of the machine. i v

The various voltages required for the operation of the processor are supplied by DC supply 54.

Motor 56 is a vacuum motor which exhausts smoke from the stylus through a filter. Limit stop 58 automatically stops the operation of the machine when the pattern on the stencil is complete. Stylus solenoid 60 lowers the stylus into the operating position when the machine is started and retracts it when the machine is turned off. The retraction of the stylus is important in order to prevent damage to the stylus when loading the drum.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. An electronic stencil cutter for reproducing an original pattern on a stencil, said stencil cutter comprismg:

.states and said capacitance means is fully charged photoresponsive means for producing an electrical output which is directly proportional to the intensity of the original pattern;

stylus control signal generator means, coupled to said photoresponsive means, for producing a pulse 5 train output wherein the pulse width is inversely proportional to the intensity of the original pattern;

. power amplifier means, coupled to said stylus control signal generator means, for receiving the output of said stylus control signal generator means and for producing a stylus current in accordance with the pulse width of the output of said stylus control signal generator means, whereby the stylus receives current, controlled by said power amplifier means, for a period of time inversely proportional to the intensity of the original pattern such that the density of the pattern on said stencil is inversely proportional to the intensity of the original pattern; and wherein said photoresponsive means 30 comprises a photomultiplier and said stylus control signal generator means comprises:

. capacitance means coupled to said photomultiplier means, wherein said capacitance means is charged by the current through said photomultiplier;

oscillator means coupled to said capacitance means, said oscillator means oscillating between two states; and

switching means coupled to said capacitance means, such that said switching means is ON when said oscillator means is in one of its two states and when said oscillator means is in the other of its two and said switching means is OFF when said oscillator means is not fully charged whereby the OFF pe- 6 riod of said switching means is inversely proportional to the current through said photomultiplier means.

2. The stencil cutter as set forth in claim 1, wherein said stylus control signal generator means further includes resistance means, connected to said capacitance means, for varying the charging rate of said capacitance means, whereby the inverse proportionality of the OFF period of said switching means to the current through said photomultiplier means is modified to become logarithmic.

3. The stencil cutter as set forth in claim 1, further including logic gate means connected between said stylus control signal generator means and said power amplifier means for preventing the OFF period of said switching means from exceeding the ON period of said switching means.

4. The stencil cutter as set forth in claim 1, wherein said oscillator means comprises an oscillator circuit in series with a capacitor.

5. The stencil cutter as set forth in claim 4, wherein said resistance means is variable and wherein the variation of said resistance means varies the density of the pattern on said stencil for a given intensity of the original pattern.

6. The stencil cutter as set forth in claim 1, wherein said capacitance means comprises the stray capacitance ofa cable connecting said photomultiplier means and said stylus control signal generator means.

7. The stencil cutter as set forth in claim 6, wherein said power amplifier means includes adjusting means for varying the stylus current whereby said stylus current may be varied in accordance with the type of sten- 

1. An electronic stencil cutter for reproducing an original pattern on a stencil, said stencil cutter comprising: a. photoresponsive means for producing an electrical output which is directly proportional to the intensity of the original pattern; b. stylus control signal generator means, coupled to said photoresponsive means, for producing a pulse train output wherein the pulse width is inversely proportional to the intensity of the original pattern; c. power amplifier means, coupled to said stylus control signal generator means, for receiving the output of said stylus control signal generator means and for producing a stylus current in accordance with the pulse width of the output of said stylus control signal generator means, whereby the stylus receives current, controlled by said power amplifier means, for a period of time inversely proportional to the intensity of the original pattern such that the density of the pattern on said stencil is inversely proportional to the intensity of the original pattern; and wherein said photoresponsive means comprises a photomultiplier and said stylus control signal generator means comprises: d. capacitance means coupled to said photomultiplier means, wherein said capacitance means is charged by the current through said photomultiplier; e. Oscillator means coupled to said capacitance means, said oscillator means oscillating between two states; and f. switching means coupled to said capacitance means, such that said switching means is ON when said oscillator means is in one of its two states and when said oscillator means is in the other of its two states and said capacitance means is fully charged, and said switching means is OFF when said oscillator means is not fully charged whereby the OFF period of said switching means is inversely proportional to the current through said photomultiplier means.
 2. The stencil cutter as set forth in claim 1, wherein said stylus control signal generator means further includes resistance means, connected to said capacitance means, for varying the charging rate of said capacitance means, whereby the inverse proportionality of the OFF period of said switching means to the current through said photomultiplier means is modified to become logarithmic.
 3. The stencil cutter as set forth in claim 1, further including logic gate means connected between said stylus control signal generator means and said power amplifier means for preventing the OFF period of said switching means from exceeding the ON period of said switching means.
 4. The stencil cutter as set forth in claim 1, wherein said oscillator means comprises an oscillator circuit in series with a capacitor.
 5. The stencil cutter as set forth in claim 4, wherein said resistance means is variable and wherein the variation of said resistance means varies the density of the pattern on said stencil for a given intensity of the original pattern.
 6. The stencil cutter as set forth in claim 1, wherein said capacitance means comprises the stray capacitance of a cable connecting said photomultiplier means and said stylus control signal generator means.
 7. The stencil cutter as set forth in claim 6, wherein said power amplifier means includes adjusting means for varying the stylus current whereby said stylus current may be varied in accordance with the type of stencil used. 